Over the decades, wireless communication systems have become more and more technologically advanced, with performance increasing in terms of smaller size, operation at higher frequencies and the accompanying increase in bandwidth, lower power consumption for a given power output, and robustness, among other factors. The trend toward better communication systems puts ever-greater demands on the manufacturers of these systems.
Today, the demands of satellite, military, and other cutting-edge digital communication systems are being met with microwave technology. In these applications, there is a need for surface-mount packaging of circuits and systems that is compact and lightweight. The demands of microwave signal processing also require a careful choice of materials to match the thermal expansion properties between mating assemblies and minimization of solder joint where possible to improve reliability. Meanwhile, factors such as size and manufacturability necessitate higher levels of integration and the reduction of discrete components in order to lower engineering costs and reduce product design cycle time.
Microwave circuits may be categorized by the material used for construction. For example, popular technologies include low temperature co-fired ceramic (LTCC), ceramic/polyamide (CP), epoxy fiberglass (FR4), fluoropolymer composites (PTFE), and mixed dielectric (MDk, a combination of FR4 and PTFE). Each technology has its strengths and addresses one or several of the issues set forth above, but no current technology addresses all of the issues.
For example, multilayer printed circuit boards using FR4, PTFE, or MDk technologies are often used to route signals to components that are mounted on the surface by way of soldered connections of conductive polymers. For these circuits, resistors can be screen-printed or etched, and may be buried. These technologies can form multifunction modules (MCM) which carry monolithic microwave integrated circuits (MMICs) and can be mounted on a motherboard.
Although FR4 has low costs associated with it and is easy to machine, it is typically not suited for microwave frequencies, due to a high loss tangent and a high correlation between the material's dielectric constant and temperature. There is also a tendency to have coefficient of thermal expansion (CTE) differentials that cause mismatches in an assembly. Even though recent developments in FR4 boards have improved electrical properties, the thermoset films used to bond the layers may limit the types of via hole connections between layers.
PTFE is a better technology than FR4 for most microwave applications. Composites having glass and ceramic often have exceptional thermal stability. Furthermore, complex microwave circuits can be fabricated using PTFE technology and the application of fusion bonding allows homogeneous multilayer assemblies to be formed. However, present methods utilizing this technology result in devices being exposed on the surfaces of these multifunction modules. Additionally, design cycle time tends to be long and involve large costs.
Another popular technology is CP, which involves the application of very thin layers of polyamide dielectric and gold metalization onto a ceramic bottom layer containing MMICs. This technology may produce circuitry an order of magnitude smaller than FR4, PTFE, or MDk, and usually works quite well at high microwave frequencies. Semiconductors may be covered with a layer of polyamide. However, as with PTFE technology, design cycles are usually relatively long and costly. Also, CTE differentials often cause mismatches with some mating assemblies.
Finally, LTCC technology, which forms multilayer structures by combining layers of ceramic and gold metalization, also works well at high microwave frequencies. Additionally, cavities can be easily formed, to allow devices to be enclosed therein, and covered with a layer of ceramic. However, as with CP technology, design cycles are usually relatively long and costly, and CTE differentials often cause mismatches with some mating assemblies.